1. Field of the Invention:
This invention relates in general to the determination of output torque delivered by an induction motor drive driven from a dc power source with an inverter.
2. Description of the Prior Art:
It is known that the input power to an induction motor drive apparatus, that is supplied power from a dc source is in accordance with the relationship EQU Power in=voltage dc * current dc (1)
where the voltage dc and current dc can be readily measured.
The output power of the motor is the input power minus all power losses in the motor drive system and the output torque of the motor is ##EQU1## An output torque sensor can be coupled with a motor shaft to measure the output torque and this permits an empirical determination of the motor drive apparatus power losses, when the input power is known and using above equation 2.
For many applications of the motor, it might not be desired to couple a torque sensor with the motor shaft to measure the output torque.
It is known to sense the ac voltage and AC current of the motor. For a three phase induction motor this has required sensing all three phase voltages and all three phase currents, or sensing two of the phase voltages and two of the phase currents and deriving the third phase voltage and current in relation to the sensed parameters. This can present a problem in relation to the variable frequency operation of the motor.